1.
For the milk mixture: Pour the condensed milk into a large microwave
safe bowl and cover tightly with plastic wrap. Microwave on low power,
stirring and replacing the plastic every 3 to 5 minutes, until slightly
darkened and thickened, 9 to 15 minutes. (I actually heated mine on the
stovetop in a saucepan on low heat for about 15-20 minutes, stirring
often.) Remove the bowl from microwave and slowly whisk in the
evaporated milk, cream, and vanilla. Let cool to room temperature.

2
For the cake: Adjust an oven rack to the middle position and heat the
oven to 325 degrees. Grease and flour a 13-by-9-inch baking pan. Whisk
the flour, baking powder, salt, and cinnamon in a large bowl. Heat the
butter and milk in a small saucepan over low heat until the butter is
melted. Set aside off the heat.

3. With an electric mixer on
medium speed (if you're using a stand mixer, use the whisk attachment),
beat the eggs in a large bowl for about 30 seconds, then slowly add the
sugar until incorporated. Increase the speed to medium-high and beat
until the egg mixture is very thick and glossy, 5 to 7 minutes. Reduce
speed to low and slowly mix in the melted butter mixture and vanilla.
Add the flour mixture in 3 additions, scraping down the bowl as
necessary, then mix on medium speed until fully incorporated, about 30
seconds. Using a rubber spatula, scrape the batter into the prepared
pan and bake until a toothpick inserted into the center comes out
clean, 30 to 35 minutes. Transfer the cake to a wire rack and let cool
for 10 minutes.

4. Using a skewer, poke holes at 1/2-inch
intervals in the top of the cake. Slowly pour the milk mixture over the
cake until completely absorbed. Let sit at room temperature for 15
minutes, then refrigerate uncovered for 3 hours or up to 24 hours.

5.
For the frosting: Remove the cake from refrigerator 30 minutes before
serving. With an electric mixer on medium speed, beat the cream, corn
syrup, and vanilla to soft peaks, 1 to 2 minutes. Frost the cake and
slice into 3-inch squares. Serve. (The assembled cake can be
refrigerated for up to 3 days.)

Remove
the label from the can of condensed milk. Poke two vent holes in the
top, by using a can opener for example. Put the can into a pot (with
the holes up), and fill up the pot with water until it is a quarter
inch or so below the top of the can. Put the pot on a stove and turn up
the heat. Let the pot and can simmer gently for about one and a half to
two hours for runny dulce de leche, or up to four hours for solid dulce
de leche. Add more water, as necessary, when the level boils down too
much.

When it's done, let it cool for a while, and then
carefully open up the can completely and eat directly (for the solid
variety) or use as a dessert spread (for the liquid variety).